Various operating circuits are used with low-pressure discharge lamps, also, for short, "fluorescent lamps". Electronic operating circuits are well known which operate the lamps with high-frequency supply, that is, supply in the tens of kilohertz range, for example 20 kHz or higher. One such circuit arrangement is described in U.S. Pat. No. 4,525,648, De Bijl et al. This circuit is directed to control the frequency and duty cycle of half-wave inverters to control the heating and firing or ignition conditions for the fluorescent lamps. It has been found that the circuit is sensitive to tolerances in the components, and that the heating, ignition and operating parameters of the lamp vary substantially if the circuit components are subjected to tolerances; furthermore, the circuit is complex, and hence expensive to build.
U.S. Pat. No. 4,647,817, Fahnrich et al, assigned to the assignee of the present application, describes a circuit arrangement in which the load circuit of a half-wave inverter switches between states for preheating of the lamps and to fire or ignite the lamps. A positive temperature coefficient resistor, also known as a cold conductor, is used to switch over a resonance capacity of a series resonant circuit after a predetermined heating time has elapsed.
This circuit works well. The positive temperature coefficient (PTC) resistor, however, introduces losses into the circuit which may rise to between 0.5 W and 1 W for each fluorescent lamp being controlled. PTC resistors require a certain cooling time, which may be from several tens of seconds to minutes, in order to ensure a gentle starting of the lamps if the lamp or lamps has been turned OFF; this is not accomplished if the lamp is re-energized before the PTC resistor has completely cooled.
U.S. Pat. No. 5,027,033, Zuchtriegel, assigned to the assignee of the present application, described a circuit arrangement which has a relay as well as a PTC resistor connected in the heating circuit of a fluorescent lamp. The PTC resistor functions as a timing element to control the relay. After the lamp has been preheated, a relay contact removes the PTC resistor from the circuit, thereby eliminating any losses in the PTC resistor. When the lamp has fired, no current will flow through the PTC resistor anymore. This has an advantage with respect to the aforementioned U.S. Pat. No. 4,647,817 since no further losses arise in the PTC resistor when the lamp is operating. The disadvantage of the PTC resistor, namely the cooling-off time, however, is not eliminated. If short ON/OFF switching cycles for the lamp or lamps are required, the lamps are not started under gentle preheating conditions. The circuit, also, requires good matching of the circuit elements in order to avoid cold-starting of the lamp, and reliable disconnection of the PTC resistor by the relay. This patent illustrates a circuit with series connected lamps.
U.S. Pat. No. 4,730,147, Kroenig, assigned to a related company of the assignee of the present application, describes a circuit to operate a low-pressure discharge lamp which includes an inverter with a series resonance circuit coupled thereto. A heater circuit to preheat the lamp electrodes has a resonance capacity which is controlled by a temperature-dependent resistor or by a threshold switch. The temperature-dependent resistor, or the threshold switch, respectively, changes the detuning between the resonance frequency and the series resonance circuit at the end of the electrode preheating phase, and decreases the switching frequency of the inverter, so that the lamp will receive the appropriate ignition or firing voltage. After the lamp has fired, a separating switch breaks the heating circuit.
The turn-off signal for the heating circuit, which breaks the heating circuit, is not directly controlled by the electrodes, but by an additional temperature-dependent resistor or the threshold switch. The threshold switch receives its control signal from a timing circuit which is not coupled to the electrodes of the lamp. Thus, preheating which is optimally matched to the lamp electrodes cannot be ensured. Use of a temperature-dependent resistor again has the disadvantage that, when short ON/OFF switching cycles for the lamp occur, the cooling off of the temperature-dependent resistor is insufficient to permit gentle, gradual starting of the lamp.